Streamflow variability controls N and P export and speciation from Alaskan coastal temperate rainforest watersheds

Abstract

The perhumid coastal temperate rainforest (PCTR) of northwestern North America is projected to become warmer and wetter in coming decades, with largely unquantified implications for the magnitude and speciation of riverine nitrogen (N) and phosphorus (P) export from PCTR ecosystems. We collected streamwater at weekly to monthly intervals for a year and intensively during two multi-day storms (one each in summer and the autumn rainy season) from streams draining three of the most common landcover types in southeast Alaska (poor fen, forested wetland and upland forest). Our goal was to investigate how seasonal and episodic (stormflows) changes in runoff influence the magnitude and species of dissolved N and P exported from PCTR watersheds. Riverine yields of total dissolved N and P ranged from 238 to 406 kg km2 year− 1 for N and 11 to 17 kg km2 year− 1 for P and were dominated by organic nutrient forms. Yields of N and P showed a varied response to runoff, with both hydrologic transport and source limitation of nutrient yields observed across the landcover types. During stormflows, log transformed ratios of dissolved inorganic N to soluble reactive P decreased from prestorm levels of ~ 1.0 to 1.5 to less than 0.3 during peak flow at all sites, illustrating that storms induce ephemeral changes in inorganic nutrient export and stoichiometry. Our findings highlight the pulsed nature of dissolved N and P export from PCTR watersheds suggesting that future changes in the seasonality and intensity of precipitation may influence the flow of terrestrial nutrients to marine ecosystems.